Phase I - II trials of hyperfractionated total body irradiation (TBI) regimens will be carried out. Initial studies will determine the maximum tolerated dose and clinical efficacy of TBI given at 8 cGy per minute from 60 Co sources in 2 fractions of 2 Gy or 3 fractions of 1.2 Gy per day. The most promising regimen will then be evaluated in a dose escalation study at dose rates of 8, 20 and 40 cGy/min using a linear accelerator and lung shielding. Phase III clinical trials will then be carried out with the most promising regimen(s).